In my serie about Highways in Thailand this is "Phetkasem road" (route 4) that begins in Bangkok Yai and goes all the way south to Sado where it connects with the "North-South Expressway" in Malaysia and has a total length of 1.254 km (according to Google Map 1.294 km) and is passing Nakhon Pathom, Cha-am, Hua Hin, Chumphon, Ranong, Phattalung and Hat Yai. It was named after the 7th Chief of Department of Highways and build in 1950. For route see Google Maps.
Phetkasem road is an intresting route because it in the ends connects to Malaysia and with that to Singapore. Phetkasem road begins at Wong Yai at the statue of King Taksin the Great and from there goes west in direction of Nakhon Pathom where you will find the first holy stupa (Chedi) of Thailand, the Phra Pathom Chedi. The Phra Pathom Chedi is the tallest chedi of Thailand with 120 meters height.
A little bit after Nakhon Pathom the route goes south in direction of Ratchaburi where you will find the Khao Ngu Stone Park and you will be crossing the Mae Klong River. A little bit after Ratchaburi route 4 is joined by route 35 and that that is the route most people nowadays will take from Bangkok to the South of Thailand.
Following Phetkasem road to Phetchaburi where you will be passing the famous Phra Nakhon Khiri. Then following Phetkasem road to the famous beach resorts Cha Am and Hua Hin, where there is a paralel road outside Hua Hin (route 37). Following Phetkasem road to Pranburi, Kuiburi and Prachuab Khiri Kan with Khao Sam Roi Yot National Park.
Then following Phetkasem road along the East Coast of South Thailand to Chumphon, where route 4 goes a bit west to go along the Thai-Burmese border and the West Coast of South Thailand to Ranong. Then following Phetkasem road passing Khao Sok and around Phang Nang Bay to Krabi and Trang before Phetkasem road goes again to the east to the West Coast of South Thailand to Phatthalung, Hat Yai and finally to Sado at the Thai-Malaysian border. Following are some photo of places you will be passing when driving or riding Phetkasem road. And at the end you will see a map of the route of Phetkasem road.